A fundamental problem exists in adsorbing low concentrations of a variety of unwanted or target substances from enclosed volume or enclosed ambient vapor phase. At minimal parts per million concentrations, adsorbing significant quantities of a variety of unwanted or target substances from the enclosed ambient atmosphere (i.e.) from the enclosed vapor phase, becomes a significant problem. Low concentration target substances exhibit pressure less than 10−5 atmospheres.
Adsorption occurs when a solid surface is exposed to a gas or liquid and the enrichment of one or more of the components in an area of the interface. The term adsorption deals with the process in which molecules accumulate in the interfacial layer. The adsorption process is accompanied by absorption, i.e. the penetration of the gas or liquid into the solid phase. The total uptake (adsorption and absorption) of gas or liquid by a solid material is sorption. At low concentrations in the enclosed volume or enclosed ambient vapor phase, there is very little physical cause, on an energetic basis, for the substances to be adsorbed. Physical adsorption or condensation of the adsorbate on a specific surface is a reversible process that occurs at a temperature lower or close to the critical temperature (i.e., the temperature at and above which vapor of the substance cannot be liquefied) of an adsorbed substance. This adsorption process which proceeds only on flat surfaces (including macroporous surfaces or pore with internal width >50 nm) of a solid can be distinguished from capillary condensation which takes place if the adsorbent has a mesoporous (2-50 nm), microporous (<2 nm) or nanoporous (a subset of porous materials, typically having large porosities [greater than 0.4], and pore diameters between 1 to 100 nm) structures. Capillary adsorbate condensation does not occur in macropores. Capillary condensation plays an important but secondary role in comparison with physical adsorption of gases by porous solids.
Adsorption theory is based mainly on the Langmuir (concept of monolayer adsorption, formed on energetically homogeneous solid surfaces) and BET (multilayer isotherm equation proposed by Brunauer, Emmett and Teller) equations, capillary condensation theory, Polanyi potential theory (adsorption potential and the characteristic adsorption curve, which are independent on the of adsorption temperature) and the DR equation (adsorption based on considerations of adsorption energies) related to the latter. The Langmuir and BET equations have distinct deviations from experimental values particularly in the range of low and high relative pressures. The divergence between theory and experimental suggest the existence of additional physical factor that influences adsorption processes; an effect resulting from interactions in the interface area. The disparity is related to the energetic heterogeneity of most real solid (polycrystalline and amorphous) adsorbents. Without wishing to be bound by any theory, it is believed that it has been experimentally shown that the concept of surface heterogeneity (besides defects on the solid surface) can be disturbances in the structure and can be caused by additives (polyethylenimine) whose presence can affect significantly the surface properties of adsorbents.
When target substances are in the very low pressure range, adsorption takes place on the most active sites on the surface or within very narrow pores. Adsorbency by a synthetic polymer material such as polyolefin, polyester, polystyrene and other such materials in the functional form of fiber, film or fabric is one example of this substantial problem. We have also found, as the boiling point of the unwanted or target substances decreases, adsorption of the gaseous substance at a constant concentration become increasingly more difficult because the gaseous substances substantially remain in the vapor phase of the enclosed volume. The molecular interactions between the gaseous substance and interfacial layer are dependent on the particular surface composition and/or the pore structure. As a molecule in a vapor phase approaches a solid surface, a balance is established between the intermolecular attractive and repulsive forces. Further, many adsorbing materials, as bulk material or in a coating, can have a small residual charge present on the surface or displays a separation of charges, i.e., a dipole, effect. Any such extant charge or dipole can inhibit the target substance approach to a surface and prevent substantial adsorption on the surface. For example, in many containers a low, but objectionable, concentration of an unwanted or target substance can form and be maintained in the container contents.